1. Field of the Invention
The present invention relates to an optically amplifying waveguide, an optical amplifier module incorporating the optically amplifying waveguide, a planar-lightwave-circuit device, and an optical communication system.
2. Description of the Background Art
Optical amplification can be performed in the C-band (1,530 to 1,565 nm) and the L-band (1,570 to 1,600 nm) by using an erbium-doped optical fiber amplifier (EDFA). On the other hand, optical amplification in other wavelength bands is also required as the broad-band use of optical communication systems increases. In recent years, researchers and engineers have been developing a technique for optically amplifying a signal lightwave in the S-band (1,460 to 1,510 nm) by using a thulium-doped optical fiber amplifier (TDFA).
In the TDFA, however, the host glass is required to be a low-phonon-energy glass such as fluoride glass. This requirement renders the TDFA insufficient in reliability. (To date, there have been no occasions where fluoride glass is commercially used.) In addition, the TDFA can only achieve the gain in a range up to 1,510 nm or so. This upper limit produces a wavelength-range width of about 20 nm, in which the gain cannot be achieved, between the optical-amplification bands covered by the conventional EDFA and the TDFA. In other words, the low-loss wavelength range of the optical fiber cannot be sufficiently utilized. Furthermore, the TDFA has an extremely complicated dynamic behavior, so that it requires a control system different from that for the EDFA.
On the other hand, another method has been proposed in which the optical amplification is performed in a wavelength range of 1,490 to 1,520 nm or so by increasing the population inversion of a silica-based Al-codoped erbium-doped optical fiber (EDF). This method is described in the published Japanese patent application Tokukai 2001-313433 and a literature entitled “Novel 1500 nm-band EDFA with discrete Raman amplifier” (E. Ishikawa et al., ECOC 2001, Postdeadline papers, pp. 48–49).
However, the EDFA incorporating a silica-based Al-codoped EDF has a strong tendency to increase the gain with increasing wavelength in a wavelength range of 1,490 to 1,520 nm or so. When this EDFA is used alone, even when an optical filter is used to equalize the gain, the relative gain variation is as high as 56%. Consequently, the gain having a small wavelength dependency cannot be achieved. As a result, a Raman amplifier must be incorporated. However, a Raman amplifier has the following drawbacks, for example. (a) It has a lower pumping efficiency than that of a rare-earth-doped optical amplifier. (b) It requires a fiber length as long as several kilometers, so that it becomes large. (c) It may deteriorate the transmission quality due to a nonlinear phenomenon and a double Rayleigh scattering in the optical fiber.